This invention relates to a process for isolating 1,5-and 1,8-dinitronaphthalene from a mixture of isomeric dinitronaphthalenes.
It is known that mixtures of isomeric dinitronaphthalenes are obtained when naphthalene is nitrated, for example with a mixture of concentrated nitric acid and sulphuric acid (Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), volume 10/1, page 494 (1971)). A number of methods are known for obtaining 1,5-and 1,8-dinitronaphtahlene from these mixtures. Thus it is possible, for example, to dissolve the mixtures in ethylene dichloride and, by fractional cooling, to deposit and separate off from the hot solution first the 1,5 component at 50.degree. C. and then, after concentrating the filtrate, the 1,8 component at room temperature (German Published Specification No. 1,618,109). Differences in the solubilities of the two dinitronaphthalenes in organic solvents are also used in two further known processes for the separation of such mixtures, the solvents used being dimethylformamide (French Pat. No. 1,320,250) and acetone (Japanese Patent Application No. 2551/66). In Chemisches Zentralblatt (1938), volume I, page 587, a process is described according to which 1,8-dinitronaphthalene is purified by recrystallisation from benzene or acetic anhydride. From Chemical Abstracts, volume 43 (1949), page 6190, a process is known which utilises the differences in the solubilities of the two dinitronapthalenes in concentrated sulphuric acid for a separation. Moreover, it is known from German Published Specification No. 1,618,109 partially to dissolve the mixture of isomers in nitric acid and then to effect fractional deposition of the components by cooling and diluting with water.